Surface acoustic wave device including an IDT formed by a metal filled in grooves on a piezoelectric substrate

ABSTRACT

A surface acoustic wave device includes a LiNbO 3  substrate and is constructed such that a reflection coefficient of an IDT is not only high but the electromechanical coupling coefficient K 2  is also high, and the range of Euler angles of the LiNbO 3  substrate which obtains a wide range of the electromechanical coupling coefficient K 2  is increased. A plurality of grooves are provided in the upper surface of the LiNbO 3  substrate, and an IDT including a plurality of electrode fingers is provided and defined by a metal material filled in the plurality of grooves, and the metal material is made of Ag, Ni, or Cr or an alloy primarily including at least one Ag, Ni, or Cr.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to a surface acoustic wave device which isused, for example, as a resonator or a band pass filter, and moreparticularly, to a surface acoustic wave device including an IDT that isdefined by a metal filled in grooves on a piezoelectric substrate.

2. Description of the Related Art

Heretofore, as a resonator and/or a band pass filter, a surface acousticwave device has been widely used. For example, in WO2006/011417A1, asurface acoustic wave device 1001 having a cross-sectional structureschematically shown in FIG. 36 is disclosed.

In the surface acoustic wave device 1001, a plurality of grooves 1002 bare provided in an upper surface 1002 a of a LiTaO₃ substrate 1002. Ametal is filled in the plurality of grooves 1002 b, and thereby an IDT1003 including a plurality of electrode fingers made of the metal isformed. A SiO₂ film 1004 is laminated so as to cover the upper surface1002 a of the LiTaO₃ substrate 1002. Since the LiTaO₃ substrate 1002 hasa negative temperature coefficient of frequency TCF, the SiO₂ film 1004having a positive temperature coefficient of frequency TCF is laminated,so that the absolute value of the temperature coefficient of frequencyTCF of the surface acoustic wave device 1001 can be decreased.

In addition, it is believed that since the IDT is defined by the metalfilled in the plurality of grooves 1002 b, a high reflection coefficientis obtained in the IDT. In particular, when the wavelength of a surfaceacoustic wave is represented by λ, and the thickness of Al filled in thegrooves 1002 b, that is, the thickness of the IDT made of Al, is set to0.04λ, the reflection coefficient per one electrode finger is 0.05, andit has been shown that as the thickness of the electrode is increased,the reflection coefficient can be increased.

In addition, in Japanese Unexamined Patent Application Publication No.2004-112748, a surface acoustic wave device shown in FIG. 37 isdisclosed. In a surface acoustic wave device 1101, an IDT 1103 isprovided on a piezoelectric substrate 1102 made of LiTaO₃ or LiNbO₃. Inaddition, a protective film 1104 is arranged so as to cover the IDT1103. On the other hand, in a region other than that in which the IDT1103 and the protective film 1104 are provided, a first insulating layer1105 made of SiO₂ is provided which has a thickness equal to that of alaminated metal film made of the IDT 1103 and the protective film 1104laminated to each other. In addition, a second insulating layer 1106made of SiO₂ is laminated so as to cover the first insulating layer1105. In this case, it has been shown that since a metal having adensity greater than that of Al is used for the IDT 1103, the absolutevalue of the reflection coefficient can be increased, and undesirableripples can be prevented.

In the surface acoustic wave device 1001 disclosed in WO2006/011417A1,it has been shown that as the thickness of the IDT made of Al isincreased, the absolute value of the reflection coefficient can beincreased. However, the inventors of the present invention discoveredthat when the absolute value of the reflection coefficient is simplyincreased, superior resonant characteristics cannot be obtained. Thatis, in the surface acoustic wave device disclosed in WO2006/011417A1,although the absolute value of the reflection coefficient can beincreased by increasing the thickness of the electrode made of Al, itwas found that since the sign of the reflection coefficient is negative,many ripples are generated in a pass band, and thus, superior resonantcharacteristics cannot be obtained.

In WO2006/011417A1, the relationship between the thickness of the IDTand the reflection coefficient is disclosed only for the case in whichan IDT is made of Al and is provided on a LiTaO₃ substrate. In addition,in paragraph [0129] of WO2006/011417A1, it has been suggested that theIDT may be made using another metal, such as Au, however, only an IDTmade of Au is disclosed.

In addition, according to Japanese Unexamined Patent ApplicationPublication No. 2004-112748, when the IDT made of a metal having adensity greater than that of Al is used, the absolute value of thereflection coefficient can be increased. However, an increase in theelectromechanical coupling coefficient of a surface acoustic wave deviceis not particularly described.

In addition, in the structure in which the IDT is defined by filling Auin the grooves provided in the above LiNbO₃ substrate, there has been aproblem in that the range of the Euler angles of a LiNbO₃ substratewhich can be used to obtain a sufficiently high electromechanicalcoupling coefficient K² is small.

SUMMARY OF THE INVENTION

To overcome the problems described above, preferred embodiments of thepresent invention provide a surface acoustic wave device in which aLiNbO₃ substrate is used as a piezoelectric substrate, the reflectioncoefficient of an IDT is not only sufficiently high but theelectromechanical coupling coefficient K² is also high, the range of theEuler angles of a LiNbO₃ substrate which can be used is relatively wide,and the degree of design freedom is increased.

According to a preferred embodiment of the present invention, a surfaceacoustic wave device includes a piezoelectric substrate including aplurality of grooves provided in an upper surface thereof and which ismade of a LiNbO₃ substrate, and an IDT having a plurality of electrodefingers made of a metal material which is filled in the plurality ofgrooves in the upper surface of the piezoelectric substrate, and themetal material is preferably selected from the group consisting of Ag,Ni, and Cr or an alloy primarily including at least one of Ag, Ni, orCr, for example.

In the surface acoustic wave device according to this preferredembodiment, when the wavelength of a surface acoustic wave isrepresented by λ, an electrode thickness of the IDT and θ of Eulerangles (0°±10°, θ, 0°±10°) of the LiNbO₃ substrate are one ofcombinations shown in Table 1. In this case, the range of the Eulerangle of the LiNbO₃ substrate which can obtain a high electromechanicalcoupling coefficient K² can be further increased.

TABLE 1 ELECTRODE ELECTRODE EULER MATERIAL THICKNESS ANGLE θ Ag 0.02λ ≦Ag ≦ 0.04λ 70° ≦ θ ≦ 145° Ag 0.04λ < Ag ≦ 0.08λ 70° ≦ θ ≦ 142° Ni 0.04λ≦ Ni ≦ 0.08λ 70° ≦ θ ≦ 153° Cr 0.02λ ≦ Cr ≦ 0.04λ 70° ≦ θ ≦ 145° Cr0.04λ < Cr ≦ 0.08λ 70° ≦ θ ≦ 152°

In addition, the surface acoustic wave device preferably furtherincludes a dielectric film which is made of SiO₂ or an inorganicmaterial primarily including SiO₂ and which covers the IDT and thepiezoelectric substrate. In this case, since the temperature coefficientof frequency of the dielectric film made of SiO₂ or an inorganicmaterial primarily including SiO₂ is a positive value, and thetemperature coefficient of frequency TCF of LiNbO₃ is a negative value,a surface acoustic wave device having a small absolute value of thetemperature coefficient of frequency TCF is provided.

In a surface acoustic wave device according to another preferredembodiment of the present invention, when the wavelength of a surfaceacoustic wave is represented by λ, a normalized film thickness of theIDT normalized by λ, a normalized film thickness of an SiO₂ film used asthe dielectric film normalized by λ, and θ of Euler angles (0°±10°, θ,0°±10°) of the LiNbO₃ substrate are one of the combinations shown in thefollowing Tables 2 to 4, Tables 5 to 7, or Tables 8 to 10.

TABLE 2 RANGE OF EULER ANGLE TO SATISFY 0.2 ≧ K² LOWER UPPER LIMIT OFLIMIT OF EULER EULER 4 ≦ FILM THICKNESS OF Ag ≦ 8 ANGLE θ ANGLE θ  0 ≦FILM THICKNESS OF SiO₂ ≦ 5 72 139  5 < FILM THICKNESS OF SiO₂ ≦ 10 75136 10 < FILM THICKNESS OF SiO₂ ≦ 15 78 128 15 < FILM THICKNESS OF SiO₂≦ 20 82 122 20 < FILM THICKNESS OF SiO₂ ≦ 25 91 115 25 < FILM THICKNESSOF SiO₂ ≦ 30 97 101 30 < FILM THICKNESS OF SiO₂ ≦ 35 — — (The filmthickness of Ag and the film thickness of SiO₂ in the table eachindicate the value of the normalized film thickness × 10².)

TABLE 3 RANGE OF EULER ANGLE TO SATISFY 0.2 ≧ K² LOWER UPPER LIMIT OFLIMIT OF EULER EULER 8 < FILM THICKNESS OF Ag ≦ 12 ANGLE θ ANGLE θ  0 ≦FILM THICKNESS OF SiO₂ ≦ 5 72 139  5 < FILM THICKNESS OF SiO₂ ≦ 10 75136 10 < FILM THICKNESS OF SiO₂ ≦ 15 78 126 15 < FILM THICKNESS OF SiO₂≦ 20 80 122 20 < FILM THICKNESS OF SiO₂ ≦ 25 81 122 25 < FILM THICKNESSOF SiO₂ ≦ 30 81 123 30 < FILM THICKNESS OF SiO₂ ≦ 35 — — (The filmthickness of Ag and the film thickness of SiO₂ in the table eachindicate the value of the normalized film thickness × 10².)

TABLE 4 RANGE OF EULER ANGLE TO SATISFY 0.2 ≧ K² LOWER UPPER LIMIT OFLIMIT OF EULER EULER 12 < FILM THICKNESS OF Ag ≦ 16 ANGLE θ ANGLE θ  0 ≦FILM THICKNESS OF SiO₂ ≦ 5 70 140  5 < FILM THICKNESS OF SiO₂ ≦ 10 70136 10 < FILM THICKNESS OF SiO₂ ≦ 15 70 126 15 < FILM THICKNESS OF SiO₂≦ 20 70 122 20 < FILM THICKNESS OF SiO₂ ≦ 25 73 122 25 < FILM THICKNESSOF SiO₂ ≦ 30 73 123 30 < FILM THICKNESS OF SiO₂ ≦ 35 73 122 (The filmthickness of Ag and the film thickness of SiO₂ in the table eachindicate the value of the normalized film thickness × 10².)

TABLE 5 RANGE OF EULER ANGLE TO SATISFY 0.2 ≧ K² LOWER UPPER LIMIT OFLIMIT OF EULER EULER 4 ≦ FILM THICKNESS OF Ni ≦ 8 ANGLE θ ANGLE θ  0 ≦FILM THICKNESS OF SiO₂ ≦ 5 70 150  5 < FILM THICKNESS OF SiO₂ ≦ 10 70147 10 < FILM THICKNESS OF SiO₂ ≦ 15 70 145 15 < FILM THICKNESS OF SiO₂≦ 20 70 138 20 < FILM THICKNESS OF SiO₂ ≦ 25 79 134 25 < FILM THICKNESSOF SiO₂ ≦ 30 93 117 30 < FILM THICKNESS OF SiO₂ ≦ 35 — — (The filmthickness of Ni and the film thickness of SiO₂ in the table eachindicate the value of the normalized film thickness × 10².)

TABLE 6 RANGE OF EULER ANGLE TO SATISFY 0.2 ≧ K² LOWER UPPER LIMIT OFLIMIT OF EULER EULER 8 < FILM THICKNESS OF Ni ≦ 12 ANGLE θ ANGLE θ  0 ≦FILM THICKNESS OF SiO₂ ≦ 5 70 157  5 < FILM THICKNESS OF SiO₂ ≦ 10 70148 10 < FILM THICKNESS OF SiO₂ ≦ 15 70 145 15 < FILM THICKNESS OF SiO₂≦ 20 72 138 20 < FILM THICKNESS OF SiO₂ ≦ 25 79 134 25 < FILM THICKNESSOF SiO₂ ≦ 30 87 129 30 < FILM THICKNESS OF SiO₂ ≦ 35 — — (The filmthickness of Ni and the film thickness of SiO₂ in the table eachindicate the value of the normalized film thickness × 10².)

TABLE 7 RANGE OF EULER ANGLE TO SATISFY 0.2 ≧ K² LOWER UPPER LIMIT OFLIMIT OF EULER EULER 12 < FILM THICKNESS OF Ni ≦ 16 ANGLE θ ANGLE θ  0 ≦FILM THICKNESS OF SiO₂ ≦ 5 73 151  5 < FILM THICKNESS OF SiO₂ ≦ 10 72151 10 < FILM THICKNESS OF SiO₂ ≦ 15 70 146 15 < FILM THICKNESS OF SiO₂≦ 20 72 140 20 < FILM THICKNESS OF SiO₂ ≦ 25 76 134 25 < FILM THICKNESSOF SiO₂ ≦ 30 81 131 30 < FILM THICKNESS OF SiO₂ ≦ 35 83 129 (The filmthickness of Ni and the film thickness of SiO₂ in the table eachindicate the value of the normalized film thickness × 10².)

TABLE 8 RANGE OF EULER ANGLE TO SATISFY 0.2 ≧ K² LOWER UPPER LIMIT OFLIMIT OF EULER EULER 4 ≦ FILM THICKNESS OF Cr ≦ 8 ANGLE θ ANGLE θ  0 ≦FILM THICKNESS OF SiO₂ ≦ 5 70 150  5 < FILM THICKNESS OF SiO₂ ≦ 10 70147 10 < FILM THICKNESS OF SiO₂ ≦ 15 70 145 15 < FILM THICKNESS OF SiO₂≦ 20 73 138 20 < FILM THICKNESS OF SiO₂ ≦ 25 80 128 25 < FILM THICKNESSOF SiO₂ ≦ 30 87 114 30 < FILM THICKNESS OF SiO₂ ≦ 35 — — (The filmthickness of Cr and the film thickness of SiO₂ in the table eachindicate the value of the normalized film thickness × 10².)

TABLE 9 RANGE OF EULER ANGLE TO SATISFY 0.2 ≧ K² LOWER UPPER LIMIT OFLIMIT OF EULER EULER 8 < FILM THICKNESS OF Cr ≦ 12 ANGLE θ ANGLE θ  0 ≦FILM THICKNESS OF SiO₂ ≦ 5 — —  5 < FILM THICKNESS OF SiO₂ ≦ 10 — — 10 <FILM THICKNESS OF SiO₂ ≦ 15 — — 15 < FILM THICKNESS OF SiO₂ ≦ 20 — — 20< FILM THICKNESS OF SiO₂ ≦ 25 — — 25 < FILM THICKNESS OF SiO₂ ≦ 30 — —30 < FILM THICKNESS OF SiO₂ ≦ 35 — — (The film thickness of Cr and thefilm thickness of SiO₂ in the table each indicate the value of thenormalized film thickness × 10².)

TABLE 10 RANGE OF EULER ANGLE TO SATISFY 0.2 ≧ K² LOWER UPPER LIMIT OFLIMIT OF EULER EULER 12 < FILM THICKNESS OF Cr ≦ 16 ANGLE θ ANGLE θ  0 ≦FILM THICKNESS OF SiO₂ ≦ 5 — —  5 < FILM THICKNESS OF SiO₂ ≦ 10 — — 10 <FILM THICKNESS OF SiO₂ ≦ 15 — — 15 < FILM THICKNESS OF SiO₂ ≦ 20 — — 20< FILM THICKNESS OF SiO₂ ≦ 25 — — 25 < FILM THICKNESS OF SiO₂ ≦ 30 — —30 < FILM THICKNESS OF SiO₂ ≦ 35 — — (The film thickness of Cr and thefilm thickness of SiO₂ in the table each indicate the value of thenormalized film thickness × 10².)

According various to preferred embodiments of the present invention, inan IDT including a plurality of electrode fingers made of a metalmaterial which is filled in grooves in an upper surface of a LiNbO₃substrate, since the metal material is made of a metal of Ag, Ni, or Cror an alloy primarily including at least one of Ag, Ni, or Cr, forexample, the reflection coefficient of the IDT is not only high but ahigh electromechanical coupling coefficient K² is also obtained.Furthermore, in order to achieve a range of the high electromechanicalcoupling coefficient K², the Euler angles of the LiNbO₃ substrate can beselected from a wide range of angles. Thus, the characteristics of thesurface acoustic wave device are improved, and the degree of designfreedom thereof is increased.

The above and other elements, features, steps, characteristics andadvantages of the present invention will become more apparent from thefollowing detailed description of the preferred embodiments withreference to the attached drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1A is a partial schematic front cross-sectional view showing animportant portion of a surface acoustic wave device according to apreferred embodiment of the present invention, and FIG. 1B is aschematic plan view of the surface acoustic wave device.

FIG. 2 is a view showing the relationship between the reflectioncoefficient and the Euler angle θ according to a preferred embodiment ofthe present invention which is obtained when Ni is used as a metalmaterial for an IDT, in which a solid line indicates results of astructure in which an SiO₂ film is laminated and a dotted line indicatesresults of a structure in which no SiO₂ film is laminated.

FIG. 3 is a view showing the relationship between the electromechanicalcoupling coefficient K² and the Euler angle θ according to a preferredembodiment of the present invention which is obtained when Ni is used asa metal material for an IDT, in which a solid line indicates results ofa structure in which an SiO₂ film is laminated and a dotted lineindicates results of a structure in which no SiO₂ film is laminated.

FIG. 4 is a view showing the relationship between the reflectioncoefficient and the Euler angle θ according to a preferred embodiment ofthe present invention which is obtained when Ag is used as a metalmaterial for an IDT, in which a solid line indicates results of astructure in which an SiO₂ film is laminated and a dotted line indicatesresults of a structure in which no SiO₂ film is laminated.

FIG. 5 is a view showing the relationship between the electromechanicalcoupling coefficient K² and the Euler angle θ according to a preferredembodiment of the present invention which is obtained when Ag is used asa metal material for an IDT, in which a solid line indicates results ofa structure in which an SiO₂ film is laminated and a dotted lineindicates results of a structure in which no SiO₂ film is laminated.

FIG. 6 is a view showing the relationship between the reflectioncoefficient and the Euler angle θ according to a preferred embodiment ofthe present invention which is obtained when Cr is used as a metalmaterial for an IDT, in which a solid line indicates results of astructure in which an SiO₂ film is laminated and a dotted line indicatesresults of a structure in which no SiO₂ film is laminated.

FIG. 7 is a view showing the relationship between the electromechanicalcoupling coefficient K² and the Euler angle θ according to a preferredembodiment of the present invention which is obtained when Cr is used asa metal material for an IDT, in which a solid line indicates results ofa structure in which an SiO₂ film is laminated and a dotted lineindicates results of a structure in which no SiO₂ film is laminated.

FIG. 8 is a view showing the relationship between the reflectioncoefficient and the Euler angle θ according to a related example whichis obtained when Al is used as a metal material for an IDT, in which asolid line indicates results of a structure in which an SiO₂ film islaminated and a dotted line indicates results of a structure in which noSiO₂ film is laminated.

FIG. 9 is a view showing the relationship between the electromechanicalcoupling coefficient K² and the Euler angle θ according to a relatedexample which is obtained when Al is used as a metal material for anIDT, in which a solid line indicates results of a structure in which anSiO₂ film is laminated and a dotted line indicates results of astructure in which no SiO₂ film is laminated.

FIG. 10 is a view showing the relationship between the reflectioncoefficient and the Euler angle θ according to a related example whichis obtained when Au is used as a metal material for an IDT, in which asolid line indicates results of a structure in which an SiO₂ film islaminated and a dotted line indicates results of a structure in which noSiO₂ film is laminated.

FIG. 11 is a view showing the relationship between the electromechanicalcoupling coefficient K² and the Euler angle θ according to a relatedexample which is obtained when Au is used as a metal material for anIDT, in which a solid line indicates results of a structure in which anSiO₂ film is laminated and a dotted line indicates results of astructure in which no SiO₂ film is laminated.

FIGS. 12A and 12B are views showing the relationship of the reflectioncoefficient with a normalized film thickness of a Ag film and the Eulerangle θ and the relationship of the electromechanical couplingcoefficient K² therewith, respectively, according to a preferredembodiment of the present invention, each of which is obtained when Agis used as a metal material for an IDT and no SiO₂ film is formed.

FIGS. 13A and 13B are views showing the relationship of the reflectioncoefficient with a normalized film thickness of a Ag film and the Eulerangle θ and the relationship of the electromechanical couplingcoefficient K² therewith, respectively, according to a preferredembodiment of the present invention, each of which is obtained when Agis used as a metal material for an IDT and a normalized film thicknessof an SiO₂ film is 0.05.

FIGS. 14A and 14B are views showing the relationship of the reflectioncoefficient with a normalized film thickness of a Ag film and the Eulerangle θ and the relationship of the electromechanical couplingcoefficient K² therewith, respectively, according to a preferredembodiment of the present invention, each of which is obtained when Agis used as a metal material for an IDT and a normalized film thicknessof an SiO₂ film is about 0.1.

FIGS. 15A and 15B are views showing the relationship of the reflectioncoefficient with a normalized film thickness of a Ag film and the Eulerangle θ and the relationship of the electromechanical couplingcoefficient K² therewith, respectively, according to a preferredembodiment of the present invention, each of which is obtained when Agis used as a metal material for an IDT and a normalized film thicknessof an SiO₂ film is about 0.15.

FIGS. 16A and 16B are views showing the relationship of the reflectioncoefficient with a normalized film thickness of a Ag film and the Eulerangle θ and the relationship of the electromechanical couplingcoefficient K² therewith, respectively, according to a preferredembodiment of the present invention, each of which is obtained when Agis used as a metal material for an IDT and a normalized film thicknessof an SiO₂ film is about 0.2.

FIGS. 17A and 17B are views showing the relationship of the reflectioncoefficient with a normalized film thickness of a Ag film and the Eulerangle θ and the relationship of the electromechanical couplingcoefficient K² therewith, respectively, according to a preferredembodiment of the present invention, each of which is obtained when Agis used as a metal material for an IDT and a normalized film thicknessof an SiO₂ film is about 0.25.

FIGS. 18A and 18B are views showing the relationship of the reflectioncoefficient with a normalized film thickness of a Ag film and the Eulerangle θ and the relationship of the electromechanical couplingcoefficient K² therewith, respectively, according to a preferredembodiment of the present invention, each of which is obtained when Agis used as a metal material for an IDT and a normalized film thicknessof an SiO₂ film is about 0.3.

FIGS. 19A and 19B are views showing the relationship of the reflectioncoefficient with a normalized film thickness of a Ag film and the Eulerangle θ and the relationship of the electromechanical couplingcoefficient K² therewith, respectively, according to a preferredembodiment of the present invention, each of which is obtained when Agis used as a metal material for an IDT and a normalized film thicknessof an SiO₂ film is about 0.35.

FIGS. 20A and 20B are views showing the relationship of the reflectioncoefficient with a normalized film thickness of a Ni film and the Eulerangle θ and the relationship of the electromechanical couplingcoefficient K² therewith, respectively, according to a preferredembodiment of the present invention, each of which is obtained when Niis used as a metal material for an IDT and no SiO₂ film is provided.

FIGS. 21A and 21B are views showing the relationship of the reflectioncoefficient with a normalized film thickness of a Ni film and the Eulerangle θ and the relationship of the electromechanical couplingcoefficient K² therewith, respectively, according to a preferredembodiment of the present invention, each of which is obtained when Niis used as a metal material for an IDT and a normalized film thicknessof an SiO₂ film is about 0.05.

FIGS. 22A and 22B are views showing the relationship of the reflectioncoefficient with a normalized film thickness of a Ni film and the Eulerangle θ and the relationship of the electromechanical couplingcoefficient K² therewith, respectively, according to a preferredembodiment of the present invention, each of which is obtained when Niis used as a metal material for an IDT and a normalized film thicknessof an SiO₂ film is about 0.1.

FIGS. 23A and 23B are views showing the relationship of the reflectioncoefficient with a normalized film thickness of a Ni film and the Eulerangle θ and the relationship of the electromechanical couplingcoefficient K² therewith, respectively, according to a preferredembodiment of the present invention, each of which is obtained when Niis used as a metal material for an IDT and a normalized film thicknessof an SiO₂ film is about 0.15.

FIGS. 24A and 24B are views showing the relationship of the reflectioncoefficient with a normalized film thickness of a Ni film and the Eulerangle θ and the relationship of the electromechanical couplingcoefficient K² therewith, respectively, according to a preferredembodiment of the present invention, each of which is obtained when Niis used as a metal material for an IDT and a normalized film thicknessof an SiO₂ film is about 0.2.

FIGS. 25A and 25B are views showing the relationship of the reflectioncoefficient with a normalized film thickness of a Ni film and the Eulerangle θ and the relationship of the electromechanical couplingcoefficient K² therewith, respectively, according to a preferredembodiment of the present invention, each of which is obtained when Niis used as a metal material for an IDT and a normalized film thicknessof an SiO₂ film is about 0.25.

FIGS. 26A and 26B are views showing the relationship of the reflectioncoefficient with a normalized film thickness of a Ni film and the Eulerangle θ and the relationship of the electromechanical couplingcoefficient K² therewith, respectively, according to a preferredembodiment of the present invention, each of which is obtained when Niis used as a metal material for an IDT and a normalized film thicknessof an SiO₂ film is about 0.3.

FIGS. 27A and 27B are views showing the relationship of the reflectioncoefficient with a normalized film thickness of a Ni film and the Eulerangle θ and the relationship of the electromechanical couplingcoefficient K² therewith, respectively, according to a preferredembodiment of the present invention, each of which is obtained when Niis used as a metal material for an IDT and a normalized film thicknessof an SiO₂ film is about 0.35.

FIGS. 28A and 28B are views showing the relationship of the reflectioncoefficient with a normalized film thickness of a Cr film and the Eulerangle θ and the relationship of the electromechanical couplingcoefficient K² therewith, respectively, according to a preferredembodiment of the present invention, each of which is obtained when Cris used as a metal material for an IDT and no SiO₂ film is provided.

FIGS. 29A and 29B are views showing the relationship of the reflectioncoefficient with a normalized film thickness of a Cr film and the Eulerangle θ and the relationship of the electromechanical couplingcoefficient K² therewith, respectively, according to a preferredembodiment of the present invention, each of which is obtained when Cris used as a metal material for an IDT and a normalized film thicknessof an SiO₂ film is about 0.05.

FIGS. 30A and 30B are views showing the relationship of the reflectioncoefficient with a normalized film thickness of a Cr film and the Eulerangle θ and the relationship of the electromechanical couplingcoefficient K² therewith, respectively, according to a preferredembodiment of the present invention, each of which is obtained when Cris used as a metal material for an IDT and a normalized film thicknessof an SiO₂ film is about 0.1.

FIGS. 31A and 31B are views showing the relationship of the reflectioncoefficient with a normalized film thickness of a Cr film and the Eulerangle θ and the relationship of the electromechanical couplingcoefficient K² therewith, respectively, according to a preferredembodiment of the present invention, each of which is obtained when Cris used as a metal material for an IDT and a normalized film thicknessof an SiO₂ film is about 0.15.

FIGS. 32A and 32B are views showing the relationship of the reflectioncoefficient with a normalized film thickness of a Cr film and the Eulerangle θ and the relationship of the electromechanical couplingcoefficient K² therewith, respectively, according to a preferredembodiment of the present invention, each of which is obtained when Cris used as a metal material for an IDT and a normalized film thicknessof an SiO₂ film is about 0.2.

FIGS. 33A and 33B are views showing the relationship of the reflectioncoefficient with a normalized film thickness of a Cr film and the Eulerangle θ and the relationship of the electromechanical couplingcoefficient K² therewith, respectively, according to a preferredembodiment of the present invention, each of which is obtained when Cris used as a metal material for an IDT and a normalized film thicknessof an SiO₂ film is about 0.25.

FIGS. 34A and 34B are views showing the relationship of the reflectioncoefficient with a normalized film thickness of a Cr film and the Eulerangle θ and the relationship of the electromechanical couplingcoefficient K² therewith, respectively, according to a preferredembodiment of the present invention, each of which is obtained when Cris used as a metal material for an IDT and a normalized film thicknessof an SiO₂ film is about 0.3.

FIGS. 35A and 35B are views showing the relationship of the reflectioncoefficient with a normalized film thickness of a Cr film and the Eulerangle θ and the relationship of the electromechanical couplingcoefficient K² therewith, respectively, according to a preferredembodiment of the present invention, each of which is obtained when Cris used as a metal material for an IDT and a normalized film thicknessof an SiO₂ film is about 0.35.

FIG. 36 is a schematic front cross-sectional view illustrating oneexample of a related surface acoustic wave device.

FIG. 37 is a partially cut-away front cross-sectional view illustratinganother example of a related surface acoustic wave device.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

Hereinafter, with reference to the drawings, preferred embodiments ofthe present invention will be described to provide a clear understandingof the present invention.

FIG. 1A is a partially schematic front cross-sectional view showing aportion in which an IDT of a surface acoustic wave device according to afirst preferred embodiment of the present invention is provided, andFIG. 1B is a schematic plan view of the above surface acoustic wavedevice.

As shown in FIG. 1A, a surface acoustic wave device 1 includes a LiNbO₃substrate 2. A plurality of grooves 2 b are provided in an upper surface2 a of the LiNbO₃ substrate 2. A metal is filled in the plurality ofgrooves 2 b, so that an IDT 3 including a plurality of electrode fingersis provided. An upper surface of this IDT 3 and the upper surface 2 a ofthe LiNbO₃ substrate 2 are flush or substantially flush with each other.

A SiO₂ film 4 is preferably arranged so as to cover the upper surface 2a and the IDT 3. However, in preferred embodiments of the presentinvention, the SiO₂ film 4 may not be provided.

As shown in FIG. 1B, the surface acoustic wave device 1 preferably is aone-port type surface acoustic wave resonator which includes the IDT 3and first and second reflectors 5 and 6 disposed at both sides of theIDT 3 in a surface acoustic wave propagation direction. In addition, thereflectors 5 and 6 are each a grating reflector in which both ends ofelectrode fingers are short-circuited to each other.

Similar to the IDT 3, the reflectors 5 and 6 are formed by filling thesame metal as that described above for the IDT 3 in grooves provided inthe upper surface 2 a of the LiNbO₃ substrate 2. Accordingly, thereflectors 5 and 6 are also flush or substantially flush with theelectrode surface and the upper surface 2 a of the LiNbO₃ substrate 2.Thus, the upper surface of the SiO₂ film 4 is substantially planar overthe entire surface acoustic wave device 1.

Although the temperature coefficient of frequency TCF of the LiNbO₃substrate 2 is a negative value, since the temperature coefficient offrequency TCF of the SiO₂ film 4 is a positive value, the overallabsolute value of the temperature coefficient of frequency TCF isdecreased. Thus, in the surface acoustic wave device 1, the change infrequency characteristics caused by changes in temperature is small.

The surface acoustic wave device 1 of this preferred embodimentpreferably is a surface acoustic wave device which utilizes an SH wave,and one of the unique features of this device is that the metal materialused for the IDT 3 is preferably Ag, Ni, or Cr or an alloy primarilyincluding at least one of Ag, Ni, or Cr, for example. A metal layer madeof another metal material, such as an adhesion layer or a diffusionprevention layer, may also be included in the IDT 3, or the IDT 3 mayhave a laminated structure including another metal layer.

Accordingly, in the surface acoustic wave device 1 of this preferredembodiment, the absolute value of the reflection coefficient of the IDT3 is not only increased but a high electromechanical couplingcoefficient K² is also obtained. In addition, as shown in the followingexperimental examples, when a surface acoustic wave device 1 having ahigh electromechanical coupling coefficient K² is obtained, the range ofthe Euler angles of a LiNbO₃ substrate which can be used issignificantly increased. Thus, the degree of design freedom isincreased. This will be described with reference to FIGS. 2 to 11.

FIGS. 8 and 9 are views showing the relationship of θ of Euler angles(0°, θ, 0°) of an LiNbO₃ substrate with the reflection coefficient orthe electromechanical coupling coefficient K² of a surface acoustic wavedevice in which although the structure thereof is similar to that of thesurface acoustic wave device 1 of the above-described preferredembodiment, an IDT electrode and reflectors are formed of Al, and aleakage surface acoustic wave is utilized.

In FIGS. 8 and 9, the results are shown of a case in which a normalizedfilm thickness of the IDT 3 made of Al normalized by a wavelength λ of asurface acoustic wave is set to about 0.04 or about 0.08. In addition,the results of a structure in which a SiO₂ film 4 having a normalizedfilm thickness of about 0.25 is provided are shown by a solid line, andthe results of a structure in which no SiO₂ film 4 is provided are shownby a dotted line.

As shown in FIG. 8, it was discovered that when Al is used as anelectrode material, the reflection coefficient is not significantlyincreased.

In addition, as shown in FIG. 9, it was discovered that when the Eulerangle θ is changed, the electromechanical coupling coefficient K² can beset to about 0.2 or greater. However, as shown in FIG. 8, when no SiO₂film 4 is provided, it was discovered that regardless of the value ofthe Euler angle θ and the film thickness of the IDT made of Al, thereflection coefficient is relatively low, such as about 0.1 or less.

On the other hand, in FIGS. 10 and 11, the results are shown of the casein which Au having a normalized film thickness of about 0.04 or about0.08 is used as the electrode material, the normalized film thicknessbeing normalized by a wavelength λ of a surface acoustic wave. That is,results are shown which are obtained when the IDT electrode 3 and thereflectors 5 and 6 shown in FIG. 1 are made of Au. FIG. 10 shows therelationship between the Euler angle θ and the reflection coefficient,and FIG. 11 shows the relationship between the Euler angle θ and theelectromechanical coupling coefficient K².

In addition, also in FIGS. 10 and 11, the results are shown by a solidline which are obtained when an SiO₂ film 4 having a normalized filmthickness 4 of about 0.25 is provided, and the results are shown by adotted line which are obtained when no SiO₂ film 4 is provided.

As shown in FIG. 10, it was discovered that as compared to the case inwhich Al is used as the electrode material shown in FIG. 8, when Au isused as the electrode material, the reflection coefficient can beincreased regardless of the Euler angle θ.

However, as shown in FIG. 11, in the structure in which no SiO₂ film 4is provided, a region having a high electromechanical couplingcoefficient K², that is, a region in which the electromechanicalcoupling coefficient K² is about 0.2 or greater, is in the range ofabout 72° to about 131° when the normalized film thickness of theelectrode made of Au is about 0.04, and when the normalized filmthickness of the electrode made of Au is about 0.08, the above region isin the range of about 85° to about 119°. Thus, when the normalized filmthickness is changed in the range of about 0.04 to about 0.08, it wasdiscovered that when the Euler angle θ is not selected in the range ofabout 85° to about 119°, the electromechanical coupling coefficient K²cannot be set to about 0.2 or greater.

In the structure in which the IDT 3 and the reflectors 5 and 6 are madeof Au and in which the SiO₂ film 4 is further provided, it wasdiscovered that in order to obtain an electromechanical couplingcoefficient K² of about 0.2 or greater, the Euler angle θ must be set inthe range of about 77° to about 117° when the normalized film thicknessof the Au film is about 0.04, and that when the normalized filmthickness thereof is about 0.08, the Euler angle θ must be set in therange of about 90° to about 114°. Thus, it was discovered that when thenormalized film thickness of the Au film is in the range of about 0.04to about 0.08, the Euler angle θ must be set in the range of about 90°to about 114°.

On the other hand, as described below, when Ag, Ni, or Cr or an alloyprimarily made of at least one of Ag, Ni, or Cr is used as the metalmaterial forming the IDT 3, the range of an Euler angle θ at which anelectromechanical coupling coefficient K² of about 0.2 or greater can beobtained is increased. Thus, the degree of design freedom of the surfaceacoustic wave device is increased.

In addition, the reason that superior characteristics are obtained whenthe electromechanical coupling coefficient K² is about 0.2 or greater isthat in a surface acoustic wave device used as a resonator or a bandpass filter, in order to obtain a bandwidth which is typically required,the electromechanical coupling coefficient K² is preferablyapproximately 0.2 or greater.

FIGS. 2 and 3 are views showing the relationship of the Euler angle θ ofa LiNbO₃ substrate with the reflection coefficient or theelectromechanical coupling coefficient K² in the case in which Ni isused as the metal material for the IDT electrode 3 and the reflectors 5and 6. In FIGS. 2 and 3, the normalized film thickness of Ni used as themetal material for the IDT 3 and the reflectors 5 and 6 is also set toabout 0.04 or about 0.08. Furthermore, also in FIGS. 2 and 3, theresults obtained when the SiO₂ film 4 is provided are shown by a solidline, and results obtained when no SiO₂ film 4 is provided are shown bya dotted line. In addition, in FIGS. 2 and 3, the results obtained whenthe IDT 3 and the reflectors 5 and 6 are provided on an upper surface ofa LiNbO₃ substrate from Ni having a normalized film thickness of about0.08 are also shown by a chain line.

As shown in FIG. 2, as in the case in which Ni is used as the metalmaterial, it was discovered that as compared to the case in which Al isused, a high reflection coefficient can be obtained regardless of therange of the Euler angle θ.

In addition, as apparent from FIG. 3, according to a comparative exampleshown by the chain line, the range of an Euler angle θ which can realizean electromechanical coupling coefficient K² of about 0.2 or greater isabout 70° to about 138° when the normalized film thickness of Ni isabout 0.04, and when the normalized film thickness thereof is about0.08, the range is about 70° to about 117°.

On the other hand, in the structure in which no SiO₂ film 4 is providedaccording to various preferred embodiments of the present invention, itwas discovered that when the film thickness of the Ni film is about0.04λ, the range of an Euler angle which can obtain an electromechanicalcoupling coefficient K² of about 0.2 or greater may be set to about 70°to about 153°, and that when the film thickness is about 0.08λ, theabove range may be set to about 70° to about 160°. Thus, it wasdiscovered that when Ni is used as the metal material, and when thethickness thereof is in the range of about 0.04λ to about 0.08λ, theEuler angle θ may be set in the range of about 70° to about 153°.Accordingly, as compared to the range of about 85° to about 119°obtained when Au is used, the range of the Euler angle θ issignificantly increased.

As in the case described above, in the structure in which the SiO₂ film4 is provided, it was discovered from FIG. 3 that when the filmthickness of Ni is about 0.04λ, the range of an Euler angle θ whichincreases the electromechanical coupling coefficient K² to about 0.2 orgreater may be set to about 78° to about 134°, and that when the filmthickness is about 0.08λ, the range may be set to about 79° to about138°. That is, it was discovered that when the thickness of Ni is in therange of about 0.04λ to about 0.08λ, the Euler angle θ may be set in therange of about 79° to about 134°. Accordingly, it was discovered that ascompared to the range of about 90° to about 114° obtained when the Aufilm is used, the range of the Euler angle θ is significantly increased.

As FIGS. 2 and 3 show, FIGS. 4 and 5 are views showing the relationshipof the Euler angle θ with the reflection coefficient or theelectromechanical coupling coefficient K². In FIGS. 4 and 5, thenormalized film thickness of Ag used as the metal material for the IDT 3and the reflectors 5 and 6 is set to about 0.02, about 0.04, or about0.08. Furthermore, in FIGS. 4 and 5, the results obtained when the SiO₂film 4 is provided are shown by a solid line, and results obtained whenno SiO₂ film 4 is provided are shown by a dotted line.

As shown in FIG. 4, in the case in which Ag is used as the metalmaterial, it was discovered that as compared to the case in which Al isused, a high reflection coefficient can be obtained regardless of therange of the Euler angle θ.

In addition, as shown in FIG. 5, in the structure in which no SiO₂ film4 is provided, it was discovered that the range of an Euler angle whichcan obtain an electromechanical coupling coefficient K² of about 0.2 orgreater may be set to about 70° to about 145° when the normalized filmthickness of the Ag film is about 0.02, that the range may be set toabout 70° to about 148° when the normalized film thickness is about0.04, and that the range may be set to about 70° to about 142° when thenormalized film thickness is about 0.08.

Accordingly, when Ag is used as the metal material, it was discoveredthat when the film thickness thereof is in the range of about 0.02λ toabout 0.04λ, the Euler angle θ may be set in the range of about 70° toabout 145°, and that when the film thickness is in the range of about0.04λ to about 0.08λ, the Euler angle θ may be set in the range of about70° to about 142°. Thus, as compared to the range of about 85° to about119° obtained when Au is used, the range of the Euler angle θ issignificantly increased.

As in the case described above, in the structure in which the SiO₂ film4 is provided, it was discovered from FIG. 5 that when the filmthickness of Ag is about 0.02λ, the range of an Euler angle θ whichobtain the electromechanical coupling coefficient K² to about 0.2 orgreater may be set to about 87° to about 130°, that when the filmthickness is about 0.04λ, the range may be set to about 91° to about115°, and that when the film thickness is about 0.08λ, the range may beset to about 81° to about 131°. That is, it was discovered that when thethickness of Ag is in the range of about 0.02λ to about 0.04λ, the Eulerangle θ may be set in the range of about 91° to about 115°, and thatwhen the thickness of Ag is in the range of about 0.04λ to about 0.08λ,the Euler angle θ may be set in the range of about 91° to about 115°.Accordingly, it was discovered that as compared to the range of about90° to about 114° obtained when the Au film is used, the range of theEuler angle θ can be increased.

As FIGS. 2 and 3 show, FIGS. 6 and 7 are views showing the relationshipof the Euler angle θ with the reflection coefficient or theelectromechanical coupling coefficient K². In FIGS. 6 and 7, thenormalized film thickness of Cr used as the metal material for the IDT 3and the reflectors 5 and 6 is set to about 0.02, about 0.04, and about0.08. Furthermore, in FIGS. 6 and 7, the results obtained when the SiO₂film 4 is provided are shown by a solid line, and results obtained whenno SiO₂ film 4 is provided are shown by a dotted line.

As shown in FIG. 6, in the case in which Cr is used as the metalmaterial, it was discovered that as compared to when Al is used, a highreflection coefficient can be obtained regardless of the range of theEuler angle θ.

In addition, as shown in FIG. 7, in the structure in which no SiO₂ film4 is provided, it was discovered that the range of an Euler angle whichcan obtain an electromechanical coupling coefficient K² of about 0.2 orgreater may be set to about 70° to about 145° when the normalized filmthickness of the Cr film is about 0.02, that the range may be set toabout 70° to about 152° when the normalized film thickness is about0.04, and that the range may be set to about 70° to about 59° when thenormalized film thickness is about 0.08.

Accordingly, when Cr is used as the metal material, it was discoveredthat when the film thickness thereof is in the range of about 0.02λ toabout 0.04λ, the Euler angle θ may be set in the range of about 70° toabout 145°, and that when the film thickness is in the range of about0.04λ to about 0.08λ, the Euler angle θ may be set in the range of about70° to about 152°. Thus, as compared to the range of about 85° to about119° obtained when Au is used, the range of the Euler angle θ issignificantly increased.

As in the case described above, when the SiO₂ film 4 is laminated, itwas discovered from FIG. 7 that when the film thickness of Cr is about0.02λ, the range of an Euler angle θ which can increase theelectromechanical coupling coefficient K² to about 0.2 or greater may beset to about 82° to about 121°, that when the film thickness is about0.04λ, the above range may be set to about 80° to about 128°, and thatwhen the film thickness is about 0.08λ, the above range may be set toabout 73° to about 140°. That is, it was discovered that when thethickness of Cr is in the range of about 0.02λ to about 0.04λ, the Eulerangle θ may be set in the range of about 82° to about 121°, and thatwhen the thickness of Cr is in the range of about 0.04λ to about 0.08λ,the Euler angle θ may be set in the range of about 80° to about 128°.Accordingly, it was discovered that as compared to the range of about90° to about 114° obtained when the Au film is used, the range of theEuler angle θ is increased.

When the results described with reference to FIGS. 2 to 7 aresummarized, a combination among the metal material used for anelectrode, the electrode thickness made of the above metal material, andthe Euler angle θ, which achieves an electromechanical couplingcoefficient K² of about 0.2 or greater, corresponds to one ofcombinations shown in Table 11 below. Table 11 shows the results of thestructure in which no SiO₂ film is provided.

TABLE 11 ELECTRODE ELECTRODE EULER MATERIAL THICKNESS ANGLE θ Ag 0.02λ ≦Ag ≦ 0.04λ 70° ≦ θ ≦ 145° Ag 0.04λ < Ag ≦ 0.08λ 70° ≦ θ ≦ 142° Ni 0.04λ≦ Ni ≦ 0.08λ 70° ≦ θ ≦ 153° Cr 0.02λ ≦ Cr ≦ 0.04λ 70° ≦ θ ≦ 145° Cr0.04λ < Cr ≦ 0.08λ 70° ≦ θ ≦ 152° Au 0.04λ ≦ Au ≦ 0.08λ 85° ≦ θ ≦ 119°

In Table 11, for comparison purpose, the case in which Au is used as themetal material is also shown.

In the above Table 11, when no SiO₂ film is provided, the range of theelectrode thickness and the range of the Euler angle θ in which theelectromechanical coupling coefficient K² is about 0.2 or greater areshown for each electrode material.

In the structure in which the SiO₂ film is provided as a dielectric filmto cover the IDT electrode, the inventors of the present invention alsotook the normalized film thickness of the SiO₂ film into considerationin addition to the electrode material and the electrode thickness, andthe range of an Euler angle θ at which the electromechanical couplingcoefficient K² is about 0.2 or greater was investigated. The resultswill be described below for each metal material.

FIGS. 12A and 12B to FIGS. 19A and 19B are views showing therelationship of the reflection coefficient with the Euler angle θ andthe relationship of the electromechanical coupling coefficient K²therewith, respectively, each of which is obtained when Ag is used asthe metal material and SiO₂ films having various normalized filmthicknesses are provided.

In addition, FIGS. 12A and 12B show the results obtained when thenormalized film thickness of the SiO₂ film is 0, that is, when no SiO₂film is provided, and FIGS. 13A to 19B show the results obtained whenthe normalized film thickness of the SiO₂ film is about 0.05, about 0.1,about 0.15, about 0.2, about 0.25, about 0.3, or about 0.35, forexample. In addition, the film of Ag used as the metal material for theIDT 3 and the reflectors 5 and 6 has various normalized film thicknessesas shown in FIGS. 12A to 19B. As shown in FIG. 4 described above, whenAg is used as the metal material, as compared to the case in which Al isused, a high reflection coefficient can be obtained regardless of therange of the Euler angle θ. In FIG. 12A to FIG. 19A, it was alsodiscovered that even when the normalized film thickness of the Ag filmis variously changed, a high reflection coefficient can be obtainedregardless of the value of the Euler angle θ.

On the other hand, as shown in FIG. 12B to FIG. 19B, it was discoveredthat when the normalized film thickness of Ag is in the range of about0.04 to about 0.08, if the range of the normalized film thickness of theSiO₂ film and the range of the Euler angle θ correspond to one ofcombinations shown in Table 12 below, the electromechanical couplingcoefficient K² can be made about 0.2 or greater. In addition, in Table12 below, the lower limit and the upper limit of the range of the Eulerangle θ are shown. For example, it indicates that when the filmthickness of the SiO₂ film is about 0.05 or less, the Euler angle θ maybe set in the range of about 72° to about 139°.

In addition, when the film thickness of the Ag film is in the range ofabout 0.08 to about 0.12, the range of the film thickness of the SiO₂film and the range of the Euler angle θ may correspond to one of thecombinations shown in Table 13 below, and when the normalized filmthickness of the Ag film is in the range of about 0.12 to about 0.16,the range of the film thickness of the SiO₂ film and the range of theEuler angle θ may correspond to one of combinations shown in Table 14below, so that the electromechanical coupling coefficient K² is about0.2 or greater. Tables 12 to 14 are based on the results shown in FIGS.12A to 19B described above.

TABLE 12 RANGE OF EULER ANGLE TO SATISFY 0.2 ≧ K² LOWER UPPER LIMIT OFLIMIT OF EULER EULER 4 ≦ FILM THICKNESS OF Ag ≦ 8 ANGLE θ ANGLE θ  0 ≦FILM THICKNESS OF SiO₂ ≦ 5 72 139  5 < FILM THICKNESS OF SiO₂ ≦ 10 75136 10 < FILM THICKNESS OF SiO₂ ≦ 15 78 128 15 < FILM THICKNESS OF SiO₂≦ 20 82 122 20 < FILM THICKNESS OF SiO₂ ≦ 25 91 115 25 < FILM THICKNESSOF SiO₂ ≦ 30 97 101 30 < FILM THICKNESS OF SiO₂ ≦ 35 — — (The filmthickness of Ag and the film thickness of SiO₂ in the table eachindicate the value of the normalized film thickness × 10².)

TABLE 13 RANGE OF EULER ANGLE TO SATISFY 0.2 ≧ K² LOWER UPPER LIMIT OFLIMIT OF EULER EULER 8 < FILM THICKNESS OF Ag ≦ 12 ANGLE θ ANGLE θ  0 ≦FILM THICKNESS OF SiO₂ ≦ 5 72 139  5 < FILM THICKNESS OF SiO₂ ≦ 10 75136 10 < FILM THICKNESS OF SiO₂ ≦ 15 78 126 15 < FILM THICKNESS OF SiO₂≦ 20 80 122 20 < FILM THICKNESS OF SiO₂ ≦ 25 81 122 25 < FILM THICKNESSOF SiO₂ ≦ 30 81 123 30 < FILM THICKNESS OF SiO₂ ≦ 35 — — (The filmthickness of Ag and the film thickness of SiO₂ in the table eachindicate the value of the normalized film thickness × 10².)

TABLE 14 RANGE OF EULER ANGLE TO SATISFY 0.2 ≧ K² LOWER UPPER LIMIT OFLIMIT OF EULER EULER 12 < FILM THICKNESS OF Ag ≦ 16 ANGLE θ ANGLE θ  0 ≦FILM THICKNESS OF SiO₂ ≦ 5 70 140  5 < FILM THICKNESS OF SiO₂ ≦ 10 70136 10 < FILM THICKNESS OF SiO₂ ≦ 15 70 126 15 < FILM THICKNESS OF SiO₂≦ 20 70 122 20 < FILM THICKNESS OF SiO₂ ≦ 25 73 122 25 < FILM THICKNESSOF SiO₂ ≦ 30 73 123 30 < FILM THICKNESS OF SiO₂ ≦ 35 73 122 (The filmthickness of Ag and the film thickness of SiO₂ in the table eachindicate the value of the normalized film thickness × 10².)

FIGS. 20A and 20B to FIGS. 27A and 27B are views showing therelationship of the reflection coefficient with the Euler angle θ andthe relationship of the electromechanical coupling coefficient K²therewith, respectively, each of which is obtained when Ni is used asthe metal material and SiO₂ films having various normalized filmthicknesses are formed.

In addition, FIGS. 20A and 20B show results obtained when the normalizedfilm thickness of the SiO₂ film is 0, that is, when no SiO₂ film isprovided, and FIGS. 21A to 27B show results obtained when the normalizedfilm thickness of the SiO₂ film is about 0.05, about 0.1, about 0.15,about 0.2, about 0.25, about 0.3, or about 0.35, for example. Inaddition, the film of Ni used as the metal material for the IDT 3 andthe reflectors 5 and 6 has various normalized film thicknesses as shownin FIGS. 20A to 27B. As shown in FIG. 2 described above, when Ni is usedas the metal material, as compared to the case in which Al is used, ahigh reflection coefficient can be obtained regardless of the range ofthe Euler angle θ. In FIG. 20A to FIG. 27A, it was also discovered thateven when the normalized film thickness of the Ni film is variouslychanged, a high reflection coefficient can be obtained regardless of thevalue of the Euler angle θ.

On the other hand, as shown in FIG. 20B to FIG. 27B, it was discoveredthat when the normalized film thickness of Ni is in the range of about0.04 to about 0.08, if the range of the normalized film thickness of theSiO₂ film and the range of the Euler angle θ correspond to one ofcombinations shown in Table 15 below, the electromechanical couplingcoefficient K² is 0.2 or greater. In addition, in Table 15, the lowerlimit and the upper limit of the range of the Euler angle θ are shown.For example, Table 15 indicates that when the film thickness of the SiO₂film is about 0.05 or less, the Euler angle θ may be set in the range ofabout 70° to about 150°.

In addition, when the film thickness of the Ni film is in the range ofabout 0.08 to about 0.12, the range of the film thickness of the SiO₂film and the range of the Euler angle θ may correspond to one ofcombinations shown in Table 16 below, and when the normalized filmthickness of the Ni film is in the range of about 0.12 to about 0.16,the range of the film thickness of the SiO₂ film and the range of theEuler angle θ may correspond to one of combinations shown in Table 17below, so that the electromechanical coupling coefficient K² is about0.2 or greater. Tables 15 to 17 are based on the results shown in FIGS.20A to 27B described above.

TABLE 15 RANGE OF EULER ANGLE TO SATISFY 0.2 ≧ K² LOWER UPPER LIMIT OFLIMIT OF EULER EULER 4 ≦ FILM THICKNESS OF Ni ≦ 8 ANGLE θ ANGLE θ  0 ≦FILM THICKNESS OF SiO₂ ≦ 5 70 150  5 < FILM THICKNESS OF SiO₂ ≦ 10 70147 10 < FILM THICKNESS OF SiO₂ ≦ 15 70 145 15 < FILM THICKNESS OF SiO₂≦ 20 70 138 20 < FILM THICKNESS OF SiO₂ ≦ 25 79 134 25 < FILM THICKNESSOF SiO₂ ≦ 30 93 117 30 < FILM THICKNESS OF SiO₂ ≦ 35 — — (The filmthickness of Ni and the film thickness of SiO₂ in the table eachindicate the value of the normalized film thickness × 10².)

TABLE 16 RANGE OF EULER ANGLE TO SATISFY 0.2 ≧ K² LOWER UPPER LIMITLIMIT OF EULER OF EULER 8 < FILM THICKNESS OF Ni ≦ 12 ANGLE θ ANGLE θ  0≦ FILM THICKNESS OF SiO₂ ≦ 5 70 157  5 < FILM THICKNESS OF SiO₂ ≦ 10 70148 10 < FILM THICKNESS OF SiO₂ ≦ 15 70 145 15 < FILM THICKNESS OF SiO₂≦ 20 72 138 20 < FILM THICKNESS OF SiO₂ ≦ 25 79 134 25 < FILM THICKNESSOF SiO₂ ≦ 30 87 129 30 < FILM THICKNESS OF SiO₂ ≦ 35 — — (The filmthickness of Ni and the film thickness of SiO₂ in the table eachindicate the value of the normalized film thickness × 10².)

TABLE 17 RANGE OF EULER ANGLE TO SATISFY 0.2 ≧ K² LOWER LIMIT UPPERLIMIT OF EULER OF EULER 12 < FILM THICKNESS OF Ni ≦ 16 ANGLE θ ANGLE θ 0 ≦ FILM THICKNESS OF SiO₂ ≦ 5 73 151  5 < FILM THICKNESS OF SiO₂ ≦ 1072 151 10 < FILM THICKNESS OF SiO₂ ≦ 15 70 146 15 < FILM THICKNESS OFSiO₂ ≦ 20 72 140 20 < FILM THICKNESS OF SiO₂ ≦ 25 76 134 25 < FILMTHICKNESS OF SiO₂ ≦ 30 81 131 30 < FILM THICKNESS OF SiO₂ ≦ 35 83 129(The film thickness of Ni and the film thickness of SiO₂ in the tableeach indicate the value of the normalized film thickness × 10².)

FIGS. 28A and 28B to FIGS. 35A and 35B are views showing therelationship of the reflection coefficient with the Euler angle θ andthe relationship of the electromechanical coupling coefficient K²therewith, respectively, each of which is obtained when Cr is used asthe metal material and SiO₂ films having various normalized filmthicknesses are provided.

In addition, FIGS. 28A and 28B show the results obtained when thenormalized film thickness of the SiO₂ film is 0, that is, when no SiO₂film is provided, and FIGS. 29A to 35B show the results obtained whenthe normalized film thickness of the SiO₂ film is about 0.05, about 0.1,about 0.15, about 0.2, about 0.25, about 0.3, or about 0.35, forexample. In addition, the film of Cr used as the metal material for theIDT 3 and the reflectors 5 and 6 has various normalized film thicknessesas shown in FIGS. 28A to 35B. As shown in FIG. 6 described above, whenCr is used as the metal material, as compared to the case in which Al isused, a high reflection coefficient can be obtained regardless of therange of the Euler angle θ. In FIG. 28A to FIG. 35B, it was alsodiscovered that even when the normalized film thickness of the Cr filmis variously changed, a high reflection coefficient can be obtainedregardless of the value of the Euler angle θ.

On the other hand, as shown in FIG. 28A to FIG. 35B, it was discoveredthat when the normalized film thickness of Cr is in the range of about0.04 to about 0.08, if the range of the normalized film thickness of theSiO₂ film and the range of the Euler angle θ correspond to one ofcombinations shown in Table 18 below, the electromechanical couplingcoefficient K² is about 0.2 or greater. In addition, in Table 18, thelower limit and the upper limit of the range of the Euler angle θ areshown. For example, FIG. 18 indicates that when the film thickness ofthe SiO₂ film is about 0.05 or less, the Euler angle θ may be set in therange of about 70° to about 150°.

In addition, when the film thickness of the Cr film is in the range ofabout 0.08 to about 0.12, the range of the film thickness of the SiO₂film and the range of the Euler angle θ may correspond to one ofcombinations shown in Table 19 below, and when the normalized filmthickness of the Cr film is in the range of about 0.12 to about 0.16,the range of the film thickness of the SiO₂ film and the range of theEuler angle θ may correspond to one of combinations shown in Table 20,so that the electromechanical coupling coefficient K² can be made 0.2 ormore as in the case described above. Tables 18 to 20 are based on theresults shown in FIGS. 28A to 35B described above.

TABLE 18 RANGE OF EULER ANGLE TO SATISFY 0.2 ≧ K² LOWER LIMIT UPPERLIMIT OF EULER OF EULER 4 ≦ FILM THICKNESS OF Cr ≦ 8 ANGLE θ ANGLE θ  0≦ FILM THICKNESS OF SiO₂ ≦ 5 70 150  5 < FILM THICKNESS OF SiO₂ ≦ 10 70147 10 < FILM THICKNESS OF SiO₂ ≦ 15 70 145 15 < FILM THICKNESS OF SiO₂≦ 20 73 138 20 < FILM THICKNESS OF SiO₂ ≦ 25 80 128 25 < FILM THICKNESSOF SiO₂ ≦ 30 87 114 30 < FILM THICKNESS OF SiO₂ ≦ 35 — — (The filmthickness of Cr and the film thickness of SiO₂ in the table eachindicate the value of the normalized film thickness × 10².)

TABLE 19 RANGE OF EULER ANGLE TO SATISFY 0.2 ≧ K² LOWER LIMIT UPPERLIMIT OF EULER OF EULER 8 < FILM THICKNESS OF Cr ≦ 12 ANGLE θ ANGLE θ  0≦ FILM THICKNESS OF SiO₂ ≦ 5 — —  5 < FILM THICKNESS OF SiO₂ ≦ 10 — — 10< FILM THICKNESS OF SiO₂ ≦ 15 — — 15 < FILM THICKNESS OF SiO₂ ≦ 20 — —20 < FILM THICKNESS OF SiO₂ ≦ 25 — — 25 < FILM THICKNESS OF SiO₂ ≦ 30 —— 30 < FILM THICKNESS OF SiO₂ ≦ 35 — — (The film thickness of Cr and thefilm thickness of SiO₂ in the table each indicate the value of thenormalized film thickness × 10².)

TABLE 20 RANGE OF EULER ANGLE TO SATISFY 0.2 ≧ K² LOWER LIMIT UPPERLIMIT OF EULER OF EULER 12 < FILM THICKNESS OF Cr ≦ 16 ANGLE θ ANGLE θ 0 ≦ FILM THICKNESS OF SiO₂ ≦ 5 — —  5 < FILM THICKNESS OF SiO₂ ≦ 10 — —10 < FILM THICKNESS OF SiO₂ ≦ 15 — — 15 < FILM THICKNESS OF SiO₂ ≦ 20 —— 20 < FILM THICKNESS OF SiO₂ ≦ 25 — — 25 < FILM THICKNESS OF SiO₂ ≦ 30— — 30 < FILM THICKNESS OF SiO₂ ≦ 35 — — (The film thickness of Cr andthe film thickness of SiO₂ in the table each indicate the value of thenormalized film thickness × 10².)

As described above, when the IDT electrode is formed, the metal materialis filled in the grooves provided in the upper surface of the LiNbO₃substrate. In this case, as the above metal material, instead of Ag, Ni,or Cr described above, an alloy primarily made of at least one of theabove metals may also be used. As the alloy described above, forexample, NiCr may be used. NiCr is an alloy primarily made of Ni or Cr.

In addition, in the experimental examples described above, although theSiO₂ film was provided, instead of the SiO₂ film, a dielectric film madeof an inorganic material primarily including an SiO₂ film, for example,may also be used. Since each film has a positive temperature coefficientof frequency, when the dielectric films described above are used incombination with an LiNbO₃ substrate having a negative temperaturecoefficient of frequency, the absolute value of the temperaturecoefficient of frequency of the surface acoustic wave device can bedecreased. That is, a surface acoustic wave device having superiortemperature characteristics can be provided.

In addition, the electrode structure of the surface acoustic wave deviceaccording to preferred embodiments of the present invention is notlimited to that shown in FIG. 1, and preferred embodiments of thepresent invention may be applied, for example, to surface acoustic waveresonators and surface acoustic wave filters which have variouselectrode structures.

In addition, according to preferred embodiments of the presentinvention, it has been shown that the Euler angles (φ, θ, ψ) of LiNbO₃are not particularly limited. However, when a Rayleigh wave or an SHwave is utilized as a surface acoustic wave, the Euler angle φ ispreferably in the range of about 0°±10°, the Euler angle θ is preferablyin the range of about 70° to about 180°, and the Euler angle ψ ispreferably in the range of about 0°±10°. That is, when the Euler anglesare set in the range of (0°±10°, 70° to 180°, 0°±10°), the Rayleigh waveand the SH wave can preferably be utilized. More particularly, in therange of (0°±10°, 90° to 180°, 0°±10°), the SH wave can more preferablybe utilized.

In addition, an LSAW wave may also be utilized, and in this case, theEuler angles may preferably be set in the range of (0°±10°, 110° to160°, 0°±10°), for example. In addition, in the above-describedpreferred embodiments, although the electrode structure of a one-porttype SAW resonator is preferably shown, the surface acoustic wave deviceaccording to various preferred embodiments of the present invention canbe widely applied to other resonator structures or other resonator typesurface acoustic wave filters.

While preferred embodiments of the present invention have been describedabove, it is to be understood that variations and modifications will beapparent to those skilled in the art without departing the scope andspirit of the present invention. The scope of the present invention,therefore, is to be determined solely by the following claims.

1. A surface acoustic wave device comprising: a piezoelectric substrateincluding a plurality of grooves provided in an upper surface thereofand which is made of an LiNbO₃ substrate; and an IDT including aplurality of electrode fingers made of a metal material which is filledin the plurality of grooves in the upper surface of the piezoelectricsubstrate; wherein the metal material is made of a metal selected fromthe group consisting of Ag, Ni, and Cr or an alloy primarily includingat least one of Ag, Ni, and Cr; and when a wavelength of a surfaceacoustic wave is represented by λ, an electrode thickness of the IDT andθ of Euler angles (0°±10°, θ, 0°±10°) of the LiNbO₃ substrate are one ofcombinations shown in Table 1: TABLE 1 ELECTRODE MATERIAL ELECTRODETHICKNESS EULER ANGLE θ Ag 0.02λ ≦ Ag ≦ 0.04λ 70° ≦ θ ≦ 145° Ag 0.04λ <Ag ≦ 0.08λ 70° ≦ θ ≦ 142° Ni 0.04λ ≦ Ni ≦ 0.08λ 70° ≦ θ ≦ 153° Cr 0.02λ≦ Cr ≦ 0.04λ 70° ≦ θ ≦ 145° Cr 0.04λ < Cr ≦ 0.08λ 70° ≦ θ ≦ 152° .


2. The surface acoustic wave device according to claim 1, furthercomprising a dielectric film which is made of SiO₂ or an inorganicmaterial primarily including SiO₂ and which is arranged to cover the IDTand the piezoelectric substrate.
 3. A surface acoustic wave devicecomprising: a piezoelectric substrate including a plurality of groovesprovided in an upper surface thereof and which is made of an LiNbO₃substrate; an IDT including a plurality of electrode fingers made of ametal material which is filled in the plurality of grooves in the uppersurface of the piezoelectric substrate; and a dielectric film which ismade of SiO₂ or an inorganic material primarily including SiO₂ and whichis arranged to cover the IDT and the piezoelectric substrate; whereinthe metal material is made of a metal selected from the group consistingof Ag, Ni, and Cr or an alloy primarily including at least one of Ag,Ni, and Cr; and when a wavelength of a surface acoustic wave isrepresented by λ, a normalized film thickness of the IDT normalized byλ, a normalized film thickness of an SiO₂ film used as the dielectricfilm normalized by λ, and θ of Euler angles (0°±10°, θ, 0°±10°) of theLiNbO₃ substrate are one of combinations shown in Tables 2 to 10: TABLE2 RANGE OF EULER ANGLE TO SATISFY 0.2 ≧ K² LOWER LIMIT UPPER LIMIT OFEULER OF EULER 4 ≦ FILM THICKNESS OF Ag ≦ 8 ANGLE θ ANGLE θ  0 ≦ FILMTHICKNESS OF SiO₂ ≦ 5 72 139  5 < FILM THICKNESS OF SiO₂ ≦ 10 75 136 10< FILM THICKNESS OF SiO₂ ≦ 15 78 128 15 < FILM THICKNESS OF SiO₂ ≦ 20 82122 20 < FILM THICKNESS OF SiO₂ ≦ 25 91 115 25 < FILM THICKNESS OF SiO₂≦ 30 97 101 30 < FILM THICKNESS OF SiO₂ ≦ 35 — — (the film thickness ofAg and the film thickness of SiO₂ in the table each indicate the valueof the normalized film thickness × 10²)

TABLE 3 RANGE OF EULER ANGLE TO SATISFY 0.2 ≧ K² LOWER LIMIT UPPER LIMITOF EULER OF EULER 8 < FILM THICKNESS OF Ag ≦ 12 ANGLE θ ANGLE θ  0 ≦FILM THICKNESS OF SiO₂ ≦ 5 72 139  5 < FILM THICKNESS OF SiO₂ ≦ 10 75136 10 < FILM THICKNESS OF SiO₂ ≦ 15 78 126 15 < FILM THICKNESS OF SiO₂≦ 20 80 122 20 < FILM THICKNESS OF SiO₂ ≦ 25 81 122 25 < FILM THICKNESSOF SiO₂ ≦ 30 81 123 30 < FILM THICKNESS OF SiO₂ ≦ 35 — — (the filmthickness of Ag and the film thickness of SiO₂ in the table eachindicate the value of the normalized film thickness × 10²)

TABLE 4 RANGE OF EULER ANGLE TO SATISFY 0.2 ≧ K² LOWER LIMIT UPPER LIMITOF EULER OF EULER 12 < FILM THICKNESS OF Ag ≦ 16 ANGLE θ ANGLE θ  0 ≦FILM THICKNESS OF SiO₂ ≦ 5 70 140  5 < FILM THICKNESS OF SiO₂ ≦ 10 70136 10 < FILM THICKNESS OF SiO₂ ≦ 15 70 126 15 < FILM THICKNESS OF SiO₂≦ 20 70 122 20 < FILM THICKNESS OF SiO₂ ≦ 25 73 122 25 < FILM THICKNESSOF SiO₂ ≦ 30 73 123 30 < FILM THICKNESS OF SiO₂ ≦ 35 73 122 (the filmthickness of Ag and the film thickness of SiO₂ in the table eachindicate the value of the normalized film thickness × 10²)

TABLE 5 RANGE OF EULER ANGLE TO SATISFY 0.2 ≧ K² LOWER LIMIT UPPER LIMITOF EULER OF EULER 4 ≦ FILM THICKNESS OF Ni ≦ 8 ANGLE θ ANGLE θ  0 ≦ FILMTHICKNESS OF SiO₂ ≦ 5 70 150  5 < FILM THICKNESS OF SiO₂ ≦ 10 70 147 10< FILM THICKNESS OF SiO₂ ≦ 15 70 145 15 < FILM THICKNESS OF SiO₂ ≦ 20 70138 20 < FILM THICKNESS OF SiO₂ ≦ 25 79 134 25 < FILM THICKNESS OF SiO₂≦ 30 93 117 30 < FILM THICKNESS OF SiO₂ ≦ 35 — — (the film thickness ofNi and the film thickness of SiO₂ in the table each indicate the valueof the normalized film thickness × 10²)

TABLE 6 RANGE OF EULER ANGLE TO SATISFY 0.2 ≧ K² LOWER LIMIT UPPER LIMITOF EULER OF EULER 8 < FILM THICKNESS OF Ni ≦ 12 ANGLE θ ANGLE θ  0 ≦FILM THICKNESS OF SiO₂ ≦ 5 70 157  5 < FILM THICKNESS OF SiO₂ ≦ 10 70148 10 < FILM THICKNESS OF SiO₂ ≦ 15 70 145 15 < FILM THICKNESS OF SiO₂≦ 20 72 138 20 < FILM THICKNESS OF SiO₂ ≦ 25 79 134 25 < FILM THICKNESSOF SiO₂ ≦ 30 87 129 30 < FILM THICKNESS OF SiO₂ ≦ 35 — — (the filmthickness of Ni and the film thickness of SiO₂ in the table eachindicate the value of the normalized film thickness × 10²)

TABLE 7 RANGE OF EULER ANGLE TO SATISFY 0.2 ≧ K² LOWER LIMIT UPPER LIMITOF EULER OF EULER 12 < FILM THICKNESS OF Ni ≦ 16 ANGLE θ ANGLE θ  0 ≦FILM THICKNESS OF SiO₂ ≦ 5 73 151  5 < FILM THICKNESS OF SiO₂ ≦ 10 72151 10 < FILM THICKNESS OF SiO₂ ≦ 15 70 146 15 < FILM THICKNESS OF SiO₂≦ 20 72 140 20 < FILM THICKNESS OF SiO₂ ≦ 25 76 134 25 < FILM THICKNESSOF SiO₂ ≦ 30 81 131 30 < FILM THICKNESS OF SiO₂ ≦ 35 83 129 (the filmthickness of Ni and the film thickness of SiO₂ in the table eachindicate the value of the normalized film thickness × 10²)

TABLE 8 RANGE OF EULER ANGLE TO SATISFY 0.2 ≧ K² LOWER LIMIT UPPER LIMITOF EULER OF EULER 4 ≦ FILM THICKNESS OF Cr ≦ 8 ANGLE θ ANGLE θ  0 ≦ FILMTHICKNESS OF SiO₂ ≦ 5 70 150  5 < FILM THICKNESS OF SiO₂ ≦ 10 70 147 10< FILM THICKNESS OF SiO₂ ≦ 15 70 145 15 < FILM THICKNESS OF SiO₂ ≦ 20 73138 20 < FILM THICKNESS OF SiO₂ ≦ 25 80 128 25 < FILM THICKNESS OF SiO₂≦ 30 87 114 30 < FILM THICKNESS OF SiO₂ ≦ 35 — — (the film thickness ofCr and the film thickness of SiO₂ in the table each indicate the valueof the normalized film thickness × 10²)

TABLE 9 RANGE OF EULER ANGLE TO SATISFY 0.2 ≧ K² LOWER LIMIT UPPER LIMITOF EULER OF EULER 8 < FILM THICKNESS OF Cr ≦ 12 ANGLE θ ANGLE θ  0 ≦FILM THICKNESS OF SiO₂ ≦ 5 — —  5 < FILM THICKNESS OF SiO₂ ≦ 10 — — 10 <FILM THICKNESS OF SiO₂ ≦ 15 — — 15 < FILM THICKNESS OF SiO₂ ≦ 20 — — 20< FILM THICKNESS OF SiO₂ ≦ 25 — — 25 < FILM THICKNESS OF SiO₂ ≦ 30 — —30 < FILM THICKNESS OF SiO₂ ≦ 35 — — (the film thickness of Cr and thefilm thickness of SiO₂ in the table each indicate the value of thenormalized film thickness × 10²)

TABLE 10 RANGE OF EULER ANGLE TO SATISFY 0.2 ≧ K² LOWER LIMIT UPPERLIMIT OF EULER OF EULER 12 < FILM THICKNESS OF Cr ≦ 16 ANGLE θ ANGLE θ 0 ≦ FILM THICKNESS OF SiO₂ ≦ 5 — —  5 < FILM THICKNESS OF SiO₂ ≦ 10 — —10 < FILM THICKNESS OF SiO₂ ≦ 15 — — 15 < FILM THICKNESS OF SiO₂ ≦ 20 —— 20 < FILM THICKNESS OF SiO₂ ≦ 25 — — 25 < FILM THICKNESS OF SiO₂ ≦ 30— — 30 < FILM THICKNESS OF SiO₂ ≦ 35 — — (the film thickness of Cr andthe film thickness of SiO₂ in the table each indicate the value of thenormalized film thickness × 10².).